Method for preventing exhaust valve seat recession

ABSTRACT

A method for preventing or inhibiting exhaust valve seat recession in a natural gas fueled internal combustion engine is disclosed. The method involves lubricating the engine with a lubricating oil composition comprising (a) a major amount of an oil of lubricating viscosity, and (b) a minor amount of a natural gas engine oil additive package, wherein the lubricating oil composition is substantially free of each of any zinc compounds and alkaline earth metal salts of a condensation product of an alkylene polyamine, an aldehyde and a substituted phenol.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to a method for preventing orinhibiting exhaust valve seat recession in natural gas fueled internalcombustion engines.

2. Description of the Related Art

Natural gas fueled engines are engines that use natural gas as a fuelsource. Lubricating oils with high resistance to oxidation, nitrationand viscosity increase are generally preferred for lubricating oils usedin natural gas engines because of the conditions related to this type ofengine.

Natural gas has a higher specific heat content than liquid hydrocarbonfuels and therefore it will burn hotter than liquid hydrocarbon fuelsunder typical conditions. In addition, since it is already a gas,natural gas does not cool the intake air by evaporation as compared toliquid hydrocarbon fuel droplets. Furthermore, many natural gas fueledengines are run either at or near stoichiometric conditions, where lessexcess air is available to dilute and cool combustion gases. As aresult, natural gas fueled engines generate higher combustion gastemperatures than engines burning liquid hydrocarbon fuels. In mostcases, natural gas fueled engines are used continuously at 70 to 100%load, whereas an engine operating in vehicular service may only spend50% of its time at full load.

This condition of running continuously near full load places severedemands on the lubricant. For example, by subjecting the lubricating toa sustained high temperature environment, the life of the lubricant isoften limited by oil oxidation processes. Also, since the rate offormation of nitrogen (NOx), increases exponentially with temperature,natural gas fueled engines may generate NO_(x) concentrations highenough to cause severe nitration of lubricating oil.

Good valve wear control is also important for keeping engine operatingcosts down and may be achieved by providing the proper amount andcomposition of ash. In addition, minimizing combustion chamber depositsand spark plug fouling are considerations in setting the ash content inthese oils. Lubricating oil ash levels are limited, so detergents mustbe carefully selected to minimize piston deposits and ring sticking.

Valve wear resistance is important to the durability of natural gasfueled engines. In general, exhaust valve recession is wear which occursat the valve and valve seat interface and is the most pronounced form ofvalve wear in natural gas fueled engines. When the valve is preventedfrom seating properly, it can cause engine roughness, poor fuel economyand excessive emissions. In order to correct excessive valve wear, acylinder head overhaul is usually required. Although natural gas fueledengines typically use very hard corrosion-resistant material for thevalve face and seat mating surface to give extended cylinder head life,it does not completely eliminate valve recession.

There is a difference in the lubricating oil requirements for naturalgas fueled engines and engines that are fueled by liquid hydrocarbonfuels. The combustion of liquid hydrocarbon fuels such as diesel fueloften results in a small amount of incomplete combustion (e.g., exhaustparticulates). In a liquid hydrocarbon fueled engine, theseincombustibles provide a small but critical degree of lubrication to theexhaust valve/seat interface, thereby ensuring the durability of bothcylinder heads and valves.

Natural gas fueled engines burn fuel that is introduced to thecombustion chamber in the gaseous phase. The combustion of natural gasfuel is often very complete, with virtually no incombustible materials.This has a significant affect on the intake and exhaust valves becausethere is no fuel-derived lubricant such as liquid droplets or soot toaid in lubrication to the exhaust valve/seat interface in a natural gasfueled engine. Therefore, the durability of the cylinder head and valveis controlled by the ash content and other properties of the lubricatingoil and its consumption rate to provide lubricant between the hot valveface and its mating seat. Too little ash or the wrong type canaccelerate valve and seat wear, while too much ash may lead to valveguttering and subsequent valve torching. Too much ash can also lead toloss of compression or detonation from combustion chamber deposits.Consequently, gas engine builders frequently specify a narrow ash rangethat they have learned provides the optimum performance. Since most gasis low in sulfur, excess ash is generally not needed to addressalkalinity requirements, and ash levels are largely optimized around theneeds of the valves. There may be exceptions to this in cases where sourgas or landfill gas is used.

Zinc dialkyldithiophosphates are a very effective additive used innatural gas engine oil additive packages for anti-wear and oxidationprotection and, for example, has been shown to contribute to the ashformed on the exhaust valve. However, it is believed that zincdialkyldithiophosphates may actually chemically react with the surfaceor form a compound with the other sources of ash that can be easilyremoved from the valve surface.

In addition, a problem associated with the use of zincdialkyldithiophosphate is that their phosphorus and sulfur derivativespoison the catalyst components of the catalytic converters. This is amajor concern as effective catalytic converters are needed to reducepollution and to meet governmental regulation designed to reduce toxicgases such as, for example, hydrocarbons, carbon monoxide and nitrogenoxides, in the internal combustion engine exhaust emissions. Suchcatalytic converters generally use a combination of catalytic metals,e.g., platinum and metal oxides, and are installed in the exhauststreams, e.g., the exhaust pipes of automobiles, to convert the toxicgases to nontoxic gases. Accordingly, it would be desirable to eliminatethe amount of zinc dialkyldithiophosphate in lubricating oils, thusreducing catalyst deactivation and hence increasing the life andeffectiveness of catalytic converters while also meeting future industrystandard proposed phosphorus and sulfur contents in the engine oil.However, simply decreasing the amount of zinc dialkyldithiophosphatepresents problems because this necessarily lowers the antiwearproperties and oxidation inhibition properties of the lubricating oil.Therefore, it is necessary to find a way to retain the antiwear andoxidation properties of the engine oils.

U.S. Pat. No. 3,798,163 (“the '163 patent”) discloses a method forcontrolling or inhibiting exhaust valve recession in natural gas fueledinternal combustion engines by maintaining a lubricating amount of alubricating oil composition on the engine components of the internalcombustion engine. The '163 patent further discloses that thelubricating oil composition contains (a) a major amount of an oil oflubricating viscosity, (b) at least one alkaline earth metal sulfonatein an amount sufficient to improve the detergency of the composition,and (c) at least one alkaline earth metal salt of a condensation productof (i) an alkylene polyamine, (ii) an aldehyde, and (iii) a substitutedphenol, wherein the alkaline earth metal salt of the condensationproduct is present in an amount sufficient to inhibit the recession ofthe engine's exhaust valves into the engine cylinder head.

U.S. Pat. No. 5,726,133 (“the '133 patent”) discloses a low ash gasengine oil comprising a major amount of a base oil of lubricatingviscosity and a minor amount sufficient to contribute a sulfated ashcontent of about 0.1 to 0.6% ash by ASTM D 874 of an additive mixturecomprising a mixture of detergents comprising at least one first alkalior alkaline earth metal salt or mixture thereof of low Total Base Number(TBN) of about 250 and less and at least one second alkali or alkalineearth metal salt or mixture thereof which is more neutral than the firstlow TBN salt. The '133 patent further discloses that the fullyformulated gas engine oil can also typically contain other standardadditives known to those skilled in the art, including antiwearadditives such as zinc dithiophosphates, dispersants, phenolic or aminicantioxidants, metal deactivators, pour point depressants, antifoamingagents, and viscosity index improvers.

U.S. Pat. No. 6,174,842 (“the '842 patent”) discloses a lubricatingcomposition containing (a) a major amount of lubricating oil, (b) anoil-soluble molybdenum compound substantially free of reactive sulfur,(c) an oil-soluble diarylamine and (d) an alkaline earth metal phenate.The '842 patent further discloses that the composition can furtherinclude a zinc dihydrocarbyl dithiophosphate as an antiwear agent. Inaddition, Oil Blend 18 disclosed in Example 2 of the '842 patentcontained an antiwear agent and was evaluated for exhaust valverecession in a Cummins Natural Gas Engine test.

It is desirable to develop improved methods for preventing or inhibitingexhaust valve recession in natural gas fueled internal combustionengines employing a lubricating oil composition free of at least anyzinc compound and which utilizes a minimum number of components.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, there isprovided a method for preventing or inhibiting exhaust valve seatrecession in a natural gas fueled engine, the method comprisinglubricating the engine with a lubricating oil composition comprising (a)a major amount of an oil of lubricating viscosity, and (b) a minoramount of a natural gas engine oil additive package, wherein thelubricating oil composition is substantially free of each of any zinccompounds and alkaline earth metal salts of a condensation product of analkylene polyamine, an aldehyde and a substituted phenol.

In accordance with a second embodiment of the present invention, thereis provided a method for enhancing the life of an exhaust valve in anatural gas fueled engine as evidenced by protection or inhibition inexhaust valve seat recession in the natural gas fueled engine, themethod comprising lubricating the engine with a lubricating oilcomposition comprising (a) a major amount of an oil of lubricatingviscosity, and (b) a minor amount of a natural gas engine oil additivepackage, wherein the lubricating oil composition is substantially freeof each of any zinc compounds and alkaline earth metal salts of acondensation product of an alkylene polyamine, an aldehyde and asubstituted phenol.

In accordance with a third embodiment of the present invention, the useof a lubricating oil composition comprising (a) a major amount of an oilof lubricating viscosity, and (b) a minor amount of a natural gas engineoil additive package, wherein the lubricating oil composition issubstantially free of each of any zinc compounds and alkaline earthmetal salts of a condensation product of an alkylene polyamine, analdehyde and a substituted phenol for the purpose of preventing orinhibiting exhaust valve seat recession in a natural gas fueled engineis provided.

In accordance with a fourth embodiment of the present invention, anatural gas engine lubricating oil composition is provided comprising(a) a major amount of an oil of lubricating viscosity, (b) one or moreashless dispersants, (c) one or more metal-containing detergents, and(d) one or more antioxidants, wherein the lubricating oil composition issubstantially free of each of any zinc compounds and alkaline earthmetal salts of a condensation product of an alkylene polyamine, analdehyde and a substituted phenol.

In accordance with a fifth embodiment of the present invention, anatural gas engine lubricating oil composition is provided comprising(a) a major amount of an oil of lubricating viscosity, (b) one or moreashless dispersants, (c) one or more metal-containing detergents, and(d) one or more antioxidants, wherein the lubricating oil composition issubstantially free of each of any zinc compounds and alkaline earthmetal salts of a condensation product of an alkylene polyamine, analdehyde and a substituted phenol, and further wherein the lubricatingoil composition has an exhaust valve seat recession reducing propertygreater than that of a corresponding lubricating oil composition inwhich a zinc compound is present therein.

In accordance with a sixth embodiment of the present invention, anatural gas engine lubricating oil composition is provided consistingessentially of (a) a major amount of an oil of lubricating viscosity,(b) one or more ashless dispersants, (c) one or more metal-containingdetergents, and (d) one or more antioxidants, wherein the lubricatingoil composition is substantially free of any zinc compounds.

In accordance with a seventh embodiment of the present invention, thereis provided a natural gas fueled internal combustion engine lubricatedwith a lubricating oil composition comprising (a) a major amount of anoil of lubricating viscosity, and (b) a minor amount of a natural gasengine oil additive package, wherein the lubricating oil composition issubstantially free of each of any zinc compounds and alkaline earthmetal salts of a condensation product of an alkylene polyamine, analdehyde and a substituted phenol.

By lubricating a natural gas fueled internal combustion engine with alubricating oil composition comprising (a) a major amount of an oil oflubricating viscosity, and (b) a minor amount of a natural gas engineoil additive package, wherein the lubricating oil composition issubstantially free of each of any zinc compounds and alkaline earthmetal salts of a condensation product of an alkylene polyamine, analdehyde and a substituted phenol, exhaust valve seat recession in thenatural gas fueled engine is advantageously prevented or inhibited ascompared to a corresponding lubricating oil composition in which a zinccompound such as a zinc dihydrocarbyl dithiophosphate compound ispresent therein. This is unexpected as zinc dihydrocarbyldithiophosphate is a known antiwear agent which contributes to the ashformed on the exhaust valve and typically used in a natural gas enginelubricating oil composition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to methods for preventing orinhibiting exhaust valve seat recession in a natural gas fueled engine.Generally, the methods involve lubricating a natural gas fueled enginewith a lubricating oil composition containing at least (a) a majoramount of an oil of lubricating viscosity, and (b) a minor amount of anatural gas engine oil additive package, wherein the lubricating oilcomposition is substantially free of each of any zinc compounds andalkaline earth metal salts of a condensation product of an alkylenepolyamine, an aldehyde and a substituted phenol. The term “substantiallyfree” as used herein shall be understood to mean only trace amounts,typically below 0.001 wt. %, based on the total weight of thelubricating oil composition, if any, of each of the zinc compounds andalkaline earth metal salts of the condensation product in thelubricating oil compositions.

The lubricating oil compositions according to the present invention foruse in natural gas fueled engines will have a sulfated ash content of nomore than about 1.5 wt. % as determined by ASTM D 874, preferably asulfated ash content of no more than about 0.95 wt. % as determined byASTM D 874 and most preferably a sulfated ash content of no more thanabout 0.5 wt. % as determined by ASTM D 874. In one embodiment, alubricating oil composition according to the present invention for usein natural gas fueled engines has a sulfated ash content of about 0.1wt. % to about 1.5 wt. % as determined by ASTM D 874, preferably about0.12 wt. % to about 0.95 wt. % as determined by ASTM D 874 and mostpreferably about 0.15 wt. % to about 0.5 wt. % as determined by ASTM D874. The lubricant ash advantageously acts as a solid lubricant toprotect the valve/seat interface in place of naturally occurring exhaustparticles in a hydrocarbon fueled engine.

In one embodiment, the lubricating oil composition of the presentinvention is substantially free of any phosphorus, e.g., a phosphoruscontent not exceeding 0.08 wt. % and more preferably not exceeding 0.05wt. %. In another embodiment, the lubricating oil composition of thepresent invention contains relatively low levels of sulfur, i.e., notexceeding 0.7 wt. %, preferably not exceeding 0.5 wt. % and morepreferably not exceeding 0.3 wt. %.

The internal combustion engines to which the present invention isapplicable may be characterized as those operated on, i.e., fueled by,natural gas and include internal combustion engines. Examples of suchengines include four cycle engines and the like. In a preferredembodiment, the internal combustion engine is a stationary engine usedin, for example, well-head gas gathering, compression, and other gaspipeline services; electrical power generation (includingco-generation); and irrigation.

The oil of lubricating viscosity for use in the lubricating oilcompositions of this invention, also referred to as a base oil, istypically present in a major amount, e.g., an amount of greater than 50wt. %, preferably greater than about 70 wt. %, more preferably fromabout 80 to about 99.5 wt. % and most preferably from about 85 to about98 wt. %, based on the total weight of the composition. The expression“base oil” as used herein shall be understood to mean a base stock orblend of base stocks which is a lubricant component that is produced bya single manufacturer to the same specifications (independent of feedsource or manufacturer's location); that meets the same manufacturer'sspecification; and that is identified by a unique formula, productidentification number, or both. The base oil for use herein can be anypresently known or later-discovered oil of lubricating viscosity used informulating lubricating oil compositions for any and all suchapplications, e.g., engine oils, marine cylinder oils, functional fluidssuch as hydraulic oils, gear oils, transmission fluids, etc.Additionally, the base oils for use herein can optionally containviscosity index improvers, e.g., polymeric alkylmethacrylates; olefiniccopolymers, e.g., an ethylene-propylene copolymer or a styrene-butadienecopolymer; and the like and mixtures thereof.

As one skilled in the art would readily appreciate, the viscosity of thebase oil is dependent upon the application. Accordingly, the viscosityof a base oil for use herein will ordinarily range from about 2 to about2000 centistokes (cSt) at 100° Centigrade (C). Generally, individuallythe base oils used as engine oils will have a kinematic viscosity rangeat 100° C. of about 2 cSt to about 30 cSt, preferably about 3 cSt toabout 16 cSt, and most preferably about 4 cSt to about 12 cSt and willbe selected or blended depending on the desired end use and theadditives in the finished oil to give the desired grade of engine oil,e.g., a lubricating oil composition having an SAE Viscosity Grade of 0W,0W-20, 0W-30, 0W-40, 0W-50, 0W-60, 5W, 5W-20, 5W-30, 5W-40, 5W-50,5W-60, 10W, 10W-20, 10W-30, 10W-40, 10W-50, 15W, 15W-20, 15W-30,15W-40,30, 40 and the like.

Base stocks may be manufactured using a variety of different processesincluding, but not limited to, distillation, solvent refining, hydrogenprocessing, oligomerization, esterification, and rerefining. Rerefinedstock shall be substantially free from materials introduced throughmanufacturing, contamination, or previous use. The base oil of thelubricating oil compositions of this invention may be any natural orsynthetic lubricating base oil. Suitable hydrocarbon synthetic oilsinclude, but are not limited to, oils prepared from the polymerizationof ethylene or from the polymerization of 1-olefins to provide polymerssuch as polyalphaolefin or PAO oils, or from hydrocarbon synthesisprocedures using carbon monoxide and hydrogen gases such as in aFischer-Tropsch process. For example, a suitable base oil is one thatcomprises little, if any, heavy fraction; e.g., little, if any, lube oilfraction of viscosity 20 cSt or higher at 100° C.

The base oil may be derived from natural lubricating oils, syntheticlubricating oils or mixtures thereof. Suitable base oil includes basestocks obtained by isomerization of synthetic wax and slack wax, as wellas hydrocracked base stocks produced by hydrocracking (rather thansolvent extracting) the aromatic and polar components of the crude.Suitable base oils include those in all API categories I, II, III, IVand V as defined in API Publication 1509, 14th Edition, Addendum I,December 1998. Group IV base oils are polyalphaolefins (PAO). Group Vbase oils include all other base oils not included in Group I, II, III,or IV. Although Group II, III and IV base oils are preferred for use inthis invention, these base oils may be prepared by combining one or moreof Group I, II, III, IV and V base stocks or base oils.

Useful natural oils include mineral lubricating oils such as, forexample, liquid petroleum oils, solvent-treated or acid-treated minerallubricating oils of the paraffinic, naphthenic or mixedparaffinic-naphthenic types, oils derived from coal or shale, animaloils, vegetable oils (e.g., rapeseed oils, castor oils and lard oil),and the like.

Useful synthetic lubricating oils include, but are not limited to,hydrocarbon oils and halo-substituted hydrocarbon oils such aspolymerized and interpolymerized olefins, e.g., polybutylenes,polypropylenes, propylene-isobutylene copolymers, chlorinatedpolybutylenes, poly(1-hexenes), poly(1-octenes), poly(1-decenes), andthe like and mixtures thereof; alkylbenzenes such as dodecylbenzenes,tetradecylbenzenes, dinonylbenzenes, di(2-ethylhexyl)-benzenes, and thelike; polyphenyls such as biphenyls, terphenyls, alkylated polyphenyls,and the like; alkylated diphenyl ethers and alkylated diphenyl sulfidesand the derivative, analogs and homologs thereof and the like.

Other useful synthetic lubricating oils include, but are not limited to,oils made by polymerizing olefins of less than 5 carbon atoms such asethylene, propylene, butylenes, isobutene, pentene, and mixturesthereof. Methods of preparing such polymer oils are well known to thoseskilled in the art.

Additional useful synthetic hydrocarbon oils include liquid polymers ofalpha olefins having the proper viscosity. Especially useful synthetichydrocarbon oils are the hydrogenated liquid oligomers of C₆ to C₁₂alpha olefins such as, for example, 1-decene trimer.

Another class of useful synthetic lubricating oils include, but are notlimited to, alkylene oxide polymers, i.e., homopolymers, interpolymers,and derivatives thereof where the terminal hydroxyl groups have beenmodified by, for example, esterification or etherification. These oilsare exemplified by the oils prepared through polymerization of ethyleneoxide or propylene oxide, the alkyl and phenyl ethers of thesepolyoxyalkylene polymers (e.g., methyl poly propylene glycol etherhaving an average molecular weight of 1,000, diphenyl ether ofpolyethylene glycol having a molecular weight of 500-1000, diethyl etherof polypropylene glycol having a molecular weight of 1,000-1,500, etc.)or mono- and polycarboxylic esters thereof such as, for example, theacetic esters, mixed C₃-C₈ fatty acid esters, or the C₁₃ oxo aciddiester of tetraethylene glycol.

Yet another class of useful synthetic lubricating oils include, but arenot limited to, the esters of dicarboxylic acids e.g., phthalic acid,succinic acid, alkyl succinic acids, alkenyl succinic acids, maleicacid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipicacid, linoleic acid dimer, malonic acids, alkyl malonic acids, alkenylmalonic acids, etc., with a variety of alcohols, e.g., butyl alcohol,hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol,diethylene glycol monoether, propylene glycol, etc. Specific examples ofthese esters include dibutyl adipate, di(2-ethylhexyl)sebacate,di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecylazelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the2-ethylhexyl diester of linoleic acid dimer, the complex ester formed byreacting one mole of sebacic acid with two moles of tetraethylene glycoland two moles of 2-ethylhexanoic acid and the like.

Esters useful as synthetic oils also include, but are not limited to,those made from carboxylic acids having from about 5 to about 12 carbonatoms with alcohols, e.g., methanol, ethanol, etc., polyols and polyolethers such as neopentyl glycol, trimethylol propane, pentaerythritol,dipentaerythritol, tripentaerythritol, and the like.

Silicon-based oils such as, for example, polyalkyl-, polyaryl-,polyalkoxy- or polyaryloxy-siloxane oils and silicate oils, compriseanother useful class of synthetic lubricating oils. Specific examples ofthese include, but are not limited to, tetraethyl silicate,tetra-isopropyl silicate, tetra-(2-ethylhexyl)silicate,tetra-(4-methyl-hexyl)silicate, tetra-(p-tert-butylphenyl)silicate,hexyl-(4-methyl-2-pentoxy)disiloxane, poly(methyl)siloxanes,poly(methylphenyl)siloxanes, and the like. Still yet other usefulsynthetic lubricating oils include, but are not limited to, liquidesters of phosphorous containing acids, e.g., tricresyl phosphate,trioctyl phosphate, diethyl ester of decane phosphionic acid, etc.,polymeric tetrahydrofurans and the like.

The lubricating oil may be derived from unrefined, refined and rerefinedoils, either natural, synthetic or mixtures of two or more of any ofthese of the type disclosed hereinabove. Unrefined oils are thoseobtained directly from a natural or synthetic source (e.g., coal, shale,or tar sands bitumen) without further purification or treatment.Examples of unrefined oils include, but are not limited to, a shale oilobtained directly from retorting operations, a petroleum oil obtaineddirectly from distillation or an ester oil obtained directly from anesterification process, each of which is then used without furthertreatment. Refined oils are similar to the unrefined oils except theyhave been further treated in one or more purification steps to improveone or more properties. These purification techniques are known to thoseof skill in the art and include, for example, solvent extractions,secondary distillation, acid or base extraction, filtration,percolation, hydrotreating, dewaxing, etc. Rerefined oils are obtainedby treating used oils in processes similar to those used to obtainrefined oils. Such rerefined oils are also known as reclaimed orreprocessed oils and often are additionally processed by techniquesdirected to removal of spent additives and oil breakdown products.

Lubricating oil base stocks derived from the hydroisomerization of waxmay also be used, either alone or in combination with the aforesaidnatural and/or synthetic base stocks. Such wax isomerate oil is producedby the hydroisomerization of natural or synthetic waxes or mixturesthereof over a hydroisomerization catalyst.

Natural waxes are typically the slack waxes recovered by the solventdewaxing of mineral oils; synthetic waxes are typically the wax producedby the Fischer-Tropsch process. Examples of useful oils of lubricatingviscosity include HVI and XHVI basestocks, such isomerized wax base oilsand UCBO (Unconventional Base Oils) base oils.

The lubricating oil compositions for use in the method of the presentinvention will also contain a minor amount of a natural gas engine oiladditive package. Typically, an additive package will contain one ormore additive components, optionally with one or more diluent oils tomake them easy to handle during shipping and storage. Suitable diluentsfor the additive package include any inert diluent, preferably an oil oflubricating viscosity, so that the additive package may be readily mixedwith lubricating oils to prepare lubricating oil compositions. Suitablelubricating oils that may be used as diluents can have a viscosity inthe range from about 35 to about 500 Saybolt Universal Seconds (SUS) at100° F. (38° C.), although any oil of lubricating viscosity may be used.If present, the one or more diluent oils will be present in an amount ofabout 10 wt. % to about 90 wt. %, based on the total weight of theadditive package. The additive package advantageously provides excellentprevention or inhibition of exhaust valve seat recession in a naturalgas fueled engine when incorporated into a lubricating oil composition.

Generally, the additive package is present in the lubricating oilcomposition from about 5 wt. % to about 15 wt. %, and preferably fromabout 6 wt. % to about 9 wt. %, based on the total weight of thelubricating oil composition. In one embodiment, the additive packagecontains at least (i) one or more ashless dispersants, (ii) one or moremetal-containing detergents, and (iii) one or more antioxidants, whereinthe lubricating oil composition is substantially free of each of anyzinc compounds, e.g., zinc dialkyl dithiophosphate compound, andalkaline earth metal salts of a condensation product of an alkylenepolyamine, an aldehyde and a substituted phenol.

The one or more ashless dispersant compounds employed in the lubricatingoil composition of the present invention are generally used to maintainin suspension insoluble materials resulting from oxidation during use,thus preventing sludge flocculation and precipitation or deposition onmetal parts. Nitrogen-containing ashless (metal-free) dispersants arebasic, and contribute to the base number or BN (as can be measured byASTM D 2896) of a lubricating oil composition to which they are added,without introducing additional sulfated ash. The term “Base Number” or“BN” as used herein refers to the amount of base equivalent tomilligrams of KOH in one gram of sample. Thus, higher BN numbers reflectmore alkaline products, and therefore a greater alkalinity. BN wasdetermined using ASTM D 2896 test. An ashless dispersant generallycomprises an oil soluble polymeric hydrocarbon backbone havingfunctional groups that are capable of associating with particles to bedispersed. Many types of ashless dispersants are known in the art.

Representative examples of ashless dispersants include, but are notlimited to, amines, alcohols, amides, or ester polar moieties attachedto the polymer backbones via bridging groups. An ashless dispersant ofthe present invention may be, for example, selected from oil solublesalts, esters, amino-esters, amides, imides, and oxazolines of longchain hydrocarbon substituted mono and dicarboxylic acids or theiranhydrides; thiocarboxylate derivatives of long chain hydrocarbons, longchain aliphatic hydrocarbons having a polyamine attached directlythereto; and Mannich condensation products formed by condensing a longchain substituted phenol with formaldehyde and polyalkylene polyamine.

Carboxylic dispersants are reaction products of carboxylic acylatingagents (acids, anhydrides, esters, etc.) comprising at least about 34and preferably at least about 54 carbon atoms with nitrogen containingcompounds (such as amines), organic hydroxy compounds (such as aliphaticcompounds including monohydric and polyhydric alcohols, or aromaticcompounds including phenols and naphthols), and/or basic inorganicmaterials. These reaction products include imides, amides, and esters.

Succinimide dispersants are a type of carboxylic dispersant. They areproduced by reacting hydrocarbyl-substituted succinic acylating agentwith organic hydroxy compounds, or with amines comprising at least onehydrogen atom attached to a nitrogen atom, or with a mixture of thehydroxy compounds and amines. The term “succinic acylating agent” refersto a hydrocarbon-substituted succinic acid or a succinic acid-producingcompound, the latter encompasses the acid itself. Such materialstypically include hydrocarbyl-substituted succinic acids, anhydrides,esters (including half esters) and halides.

Succinic-based dispersants have a wide variety of chemical structures.One class of succinic-based dispersants may be represented by theformula:

wherein each R¹ is independently a hydrocarbyl group, such as apolyolefin-derived group. Typically the hydrocarbyl group is an alkylgroup, such as a polyisobutyl group. Alternatively expressed, the R¹groups can contain about 40 to about 500 carbon atoms, and these atomsmay be present in aliphatic forms. R² is an alkylene group, commonly anethylene (C₂H₄) group. Examples of succinimide dispersants include thosedescribed in, for example, U.S. Pat. Nos. 3,172,892, 4,234,435 and6,165,235.

The polyalkenes from which the substituent groups are derived aretypically homopolymers and interpolymers of polymerizable olefinmonomers of 2 to about 16 carbon atoms, and usually 2 to 6 carbon atoms.The amines which are reacted with the succinic acylating agents to formthe carboxylic dispersant composition can be monoamines or polyamines.

Succinimide dispersants are referred to as such since they normallycontain nitrogen largely in the form of imide functionality, althoughthe amide functionality may be in the form of amine salts, amides,imidazolines as well as mixtures thereof. To prepare a succinimidedispersant, one or more succinic acid-producing compounds and one ormore amines are heated and typically water is removed, optionally in thepresence of a substantially inert organic liquid solvent/diluent. Thereaction temperature can range from about 80° C. up to the decompositiontemperature of the mixture or the product, which typically falls betweenabout 100° C. to about 300° C. Additional details and examples ofprocedures for preparing the succinimide dispersants of the presentinvention include those described in, for example, U.S. Pat. Nos.3,172,892, 3,219,666, 3,272,746, 4,234,435, 6,165,235 and 6,440,905.

Suitable ashless dispersants may also include amine dispersants, whichare reaction products of relatively high molecular weight aliphatichalides and amines, preferably polyalkylene polyamines. Examples of suchamine dispersants include those described in, for example, U.S. Pat.Nos. 3,275,554, 3,438,757, 3,454,555 and 3,565,804.

Suitable ashless dispersants may further include “Mannich dispersants,”which are reaction products of alkyl phenols in which the alkyl groupcontains at least about 30 carbon atoms with aldehydes (especiallyformaldehyde) and amines (especially polyalkylene polyamines). Examplesof such dispersants include those described in, for example, U.S. Pat.Nos. 3,036,003, 3,586,629. 3,591,598 and 3,980.569.

Suitable ashless dispersants may also be post-treated ashlessdispersants such as post-treated succinimides, e.g., post-treatmentprocesses involving borate or ethylene carbonate as disclosed in, forexample, U.S. Pat. Nos. 4,612,132 and 4,746,446; and the like as well asother post-treatment processes. The carbonate-treated alkenylsuccinimide is a polybutene succinimide derived from polybutenes havinga molecular weight of about 450 to about 3000, preferably from about 900to about 2500, more preferably from about 1300 to about 2400, and mostpreferably from about 2000 to about 2400, as well as mixtures of thesemolecular weights. Preferably, it is prepared by reacting, underreactive conditions, a mixture of a polybutene succinic acid derivative,an unsaturated acidic reagent copolymer of an unsaturated acidic reagentand an olefin, and a polyamine, such as disclosed in U.S. Pat. No.5,716,912, the contents of which are incorporated herein by reference.

Suitable ashless dispersants may also be polymeric, which areinterpolymers of oil-solubilizing monomers such as decyl methacrylate,vinyl decyl ether and high molecular weight olefins with monomerscontaining polar substitutes. Examples of polymeric dispersants includethose described in, for example, U.S. Pat. Nos. 3,329,658; 3,449,250 and3,666,730.

In a preferred embodiment of the present invention, an ashlessdispersant for use in the lubricating oil composition is abis-succinimide derived from a polyisobutenyl group having a numberaverage molecular weight of about 700 to about 2300. The dispersant(s)for use in the lubricating oil compositions of the present invention arepreferably non-polymeric (e g., are mono- or bis-succinimides).

Generally, the one or more ashless dispersants are present in thelubricating oil composition in an amount ranging from about 1 to about 8wt. %, and preferably from about 1.5 to about 6 wt. %, based on thetotal weight of the lubricating oil composition.

The one or more metal-containing detergent compounds employed in thelubricating oil composition of the present invention functions both as adetergent to reduce or remove deposits and as an acid neutralizer orrust inhibitor, thereby reducing wear and corrosion and extending enginelife. Detergents generally comprise a polar head with long hydrophobictail, with the polar head comprising a metal salt of an acid organiccompound.

The lubricating oil composition according to the present invention maycontain one or more detergents, which are normally salts, and especiallyoverbased salts. Overbased salts, or overbased materials, are singlephase, homogeneous Newtonian systems characterized by a metal content inexcess of that which would be present according to the stoichiometry ofthe metal and the particular acidic organic compound reacted with themetal. The overbased materials are prepared by reacting an acidicmaterial (typically an inorganic acid or lower carboxylic acid such ascarbon dioxide) with a mixture comprising an acidic organic compound, ina reaction medium comprising at least one inert, organic solvent (suchas mineral oil, naphtha, toluene, xylene) in the presence of astoichiometric excess of a metal base and a promoter.

Useful acidic organic compounds for making the overbased compositionsinclude carboxylic acids, sulfonic acids, phosphorus-containing acids,phenols and mixtures thereof. Preferably, the acidic organic compoundsare carboxylic acids or sulfonic acids and hydrocarbyl-substitutedsalicylic acids.

Carboxylate detergents, e.g., salicylates, can be prepared by reactingan aromatic carboxylic acid with an appropriate metal compound such asan oxide or hydroxide. Neutral or overbased products may then beobtained by methods well known in the art. The aromatic moiety of thearomatic carboxylic acid can contain one or more heteroatoms such asnitrogen and oxygen. Preferably, the moiety contains only carbon atoms.More preferably, the moiety contains six or more carbon atoms, such as abenzene moiety. The aromatic carboxylic acid may contain one or morearomatic moieties, such as one or more benzene rings, optionally fusedtogether or otherwise connected via alkylene bridges. Representativeexamples of aromatic carboxylic acids include salicylic acids andsulfurized derivatives thereof such as hydrocarbyl substituted salicylicacid and derivatives thereof. Processes for sulfurizing, for example, ahydrocarbyl-substituted salicylic acid, are known to those skilled inthe art. Salicylic acids are typically prepared by carboxylation, forexample, by the Kolbe-Schmitt process, of phenoxides. In that case,salicylic acids are generally obtained in a diluent in admixture with anuncarboxylated phenol.

Metal salts of phenols and sulfurized phenols are prepared by reactionwith an appropriate metal compound such as an oxide or hydroxide.Neutral or overbased products may be obtained by methods well known inthe art. For example, sulfurized phenols may be prepared by reacting aphenol with sulfur or a sulfur-containing compound such as hydrogensulfide, sulfur monohalide or sulfur dihalide, to form products that aremixtures of compounds in which 2 or more phenols are bridged bysulfur-containing bridges.

The metal compounds useful in making the overbased salts are generallyany Group I or Group II metal compounds in the Periodic Table of theElements. Preferably, the metal compounds are Group II metals andinclude Group IIa alkaline earth metals (e.g., magnesium, calcium,strontium, barium) as well as Group IIb metals such as zinc or cadmium.Preferably, the Group II metals are magnesium, calcium, barium, or zinc,more preferably magnesium or calcium, and most preferably calcium.

Examples of the overbased detergents include, but are not limited to,calcium sulfonates, calcium phenates, calcium salicylates, calciumstearates and mixtures thereof. Overbased detergents suitable for use inthe lubricating oil compositions of the present invention may be lowoverbased, e.g., an overbased detergent having a BN below about 100. TheBN of such a low-overbased detergent may be from about 5 to about 50, orfrom about 10 to about 30, or from about 15 to about 20. Alternatively,the overbased detergents suitable for use in the lubricating oilcompositions of the present invention may be high overbased (e.g., anoverbased detergent having a BN above about 100). The BN of such ahigh-overbased detergent may be from about 100 to about 450, or fromabout 200 to about 350, or from about 250 to about 280. A low-overbasedcalcium sulfonate detergent with a BN of about 17 and a high-overbasedsulfurized calcium phenate with a BN of about 120 are two exemplaryoverbased detergents for use in the lubricating oil compositions of thepresent invention.

The lubricating oil compositions according to the present invention maycontain more than one overbased detergent, which may be all low-BNdetergents, all high-BN detergents, or a mixture thereof. For example,the lubricating oil compositions of the present invention may contain afirst metal-containing detergent which is an overbased alkaline earthmetal sulfonate or phenate detergent having a BN of about 100 to about450 and a second metal-containing detergent which is an overbasedalkaline earth metal sulfonate or phenate detergent having a BN of about10 to about 50.

Suitable detergents for use in the lubricating oil compositions alsoinclude “hybrid” detergents such as, for example, phenate/salicylates,sulfonate/phenates, sulfonate/salicylates,sulfonates/phenates/salicylates, and the like. Examples of hybriddetergents include those described in, for example, U.S. Pat. Nos.6,153,565, 6,281,179, 6,429,178, and 6,429,179.

Generally, the one or more metal-containing detergents are present inthe lubricating oil composition in an amount ranging from about 0.5 toabout 8.5 wt. %, and preferably from about 1 to about 6 wt. %, based onthe total weight of the lubricating oil composition. Where twometal-containing detergents are employed, the first metal-containingdetergent is present in the lubricating oil composition in an amountranging from about 0.5 to about 5 wt. %, and preferably from about 1 toabout 3 wt. %, and the second metal-containing detergent is present inthe lubricating oil composition in an amount ranging from about 0.1 toabout 1.0 wt. %, and preferably from about 0.2 to about 0.5 wt. %, basedon the total weight of the lubricating oil composition.

The one or more antioxidant compounds employed in the lubricating oilcomposition of the present invention reduce the tendency of base stocksto deteriorate in service, which deterioration can be evidenced by theproducts of oxidation such as sludge and varnish-like deposits on themetal surfaces and by viscosity growth. Such oxidation inhibitorsinclude hindered phenols, ashless oil soluble phenates and sulfurizedphenates, diphenylamines, alkyl-substituted phenyl and naphthylaminesand the like and mixtures thereof. Diphenyamine-type oxidationinhibitors include, but are not limited to, alkylated diphenylamine,phenyl-α-naphthylamine, and alkylated-α-naphthylmine.

Generally, the one or more antioxidant compounds are present in thelubricating oil composition in an amount ranging from about 0.1 to about3 wt. %, and preferably from about 0.2 to about 2.5 wt. %, based on thetotal weight of the lubricating oil composition.

The lubricating oil compositions of the present invention can beconveniently prepared by simply blending or mixing the additive package,optionally with other additives, with the oil of lubricating viscosity.The additive package may also be preblended as a concentrate in theappropriate ratios to facilitate blending of a lubricating compositioncontaining the desired concentration of additives. The additive packageis blended with the base oil using a concentration at which they areboth soluble in the oil and compatible with other additives in thedesired finished lubricating oil. Compatibility in this instancegenerally means that the present compounds as well as being oil solublein the applicable treat rate also do not cause other additives toprecipitate under normal conditions. Suitable oilsolubility/compatibility ranges for a given compound of lubricating oilformulation can be determined by those having ordinary skill in the artusing routine solubility testing procedures. For example, precipitationfrom a formulated lubricating oil composition at ambient conditions(about 20° C. to 25° C.) can be measured by either actual precipitationfrom the oil composition or the formulation of a “cloudy” solution whichevidences formation of insoluble wax particles.

As previously stated, the lubricating oil compositions described hereinare substantially free of any zinc compounds and alkaline earth metalsalts of a condensation product of an alkylene polyamine, an aldehydeand a substituted phenol. In one embodiment, the lubricating oilcompositions are also substantially free of any molybdenum-containingcompounds. The alkylene polyamines of the condensation product can thefollowing structure NH₂[R(R)—NH]_(n)H wherein R is an alkylene radicalcontaining from about 2 about 6 carbon atoms, and n is an integer from 1to about 10. Typical alkylene polyamines include diethylenetriamine,triethylenetetramine, tetraethylenepentamine and the like. The aldehydesare generally aliphatic aldehydes which contain from one to about 3carbon atoms per molecule. The substituted phenols are the alkylatedmonohydric phenols having at least one alkyl group of sufficient lengthto impart oil-solubility to the condensation products. Representativealkyl phenols are those in which the alkyl group contains from about 4to about 24 carbon atoms, and preferably those having from about 8 toabout 24 carbon atoms, such as, for example, n-amyl phenol,diamylphenol, octyl phenol, nonyl phenol, p-ter-octyl phenol, a mixtureof phenols, wax alkylated phenols and the like.

The lubricating oil compositions for use in the method of the presentinvention may also contain other conventional additives for impartingauxiliary functions to give a finished lubricating oil composition inwhich these additives are dispersed or dissolved. For example, thelubricating oil compositions may be blended with antiwear agents otherthan zinc-containing antiwear agents such as zinc dialkyldithiophosphate, rust inhibitors, dehazing agents, demulsifying agents,metal deactivating agents, friction modifiers, pour point depressants,antifoaming agents, co-solvents, package compatibilisers,corrosion-inhibitors, dyes, extreme pressure agents and the like andmixtures thereof. A variety of the additives are known and commerciallyavailable. These additives, or their analogous compounds, can beemployed for the preparation of the lubricating oil compositions of theinvention by the usual blending procedures.

Examples of rust inhibitors include, but are not limited to, nonionicpolyoxyalkylene agents, e.g., polyoxyethylene lauryl ether,polyoxyethylene higher alcohol ether, polyoxyethylene nonylphenyl ether,polyoxyethylene octylphenyl ether, polyoxyethylene octyl stearyl ether,polyoxyethylene oleyl ether, polyoxyethylene sorbitol monostearate,polyoxyethylene sorbitol monooleate, and polyethylene glycol monooleate;stearic acid and other fatty acids; dicarboxylic acids; metal soaps;fatty acid amine salts; metal salts of heavy sulfonic acid; partialcarboxylic acid ester of polyhydric alcohol; phosphoric esters;(short-chain) alkenyl succinic acids; partial esters thereof andnitrogen-containing derivatives thereof; synthetic alkarylsulfonates,e.g., metal dinonylnaphthalene sulfonates; and the like and mixturesthereof.

Examples of friction modifiers include, but are not limited to,alkoxylated fatty amines; borated fatty epoxides; fatty phosphites,fatty epoxides, fatty amines, borated alkoxylated fatty amines, metalsalts of fatty acids, fatty acid amides, glycerol esters, boratedglycerol esters; and fatty imidazolines as disclosed in U.S. Pat. No.6,372,696, the contents of which are incorporated by reference herein;friction modifiers obtained from a reaction product of a C₄ to C₇₅,preferably a C₆ to C₂₄, and most preferably a C₆ to C₂₀, fatty acidester and a nitrogen-containing compound selected from the groupconsisting of ammonia, and an alkanolamine and the like and mixturesthereof.

Examples of antifoaming agents include, but are not limited to, polymersof alkyl methacrylate; polymers of dimethylsilicone and the like andmixtures thereof.

Each of the foregoing additives, when used, is used at a functionallyeffective amount to impart the desired properties to the lubricant.Thus, for example, if an additive is a friction modifier, a functionallyeffective amount of this friction modifier would be an amount sufficientto impart the desired friction modifying characteristics to thelubricant. Generally, the concentration of each of these additives, whenused, ranges from about 0.001% to about 20% by weight, and in oneembodiment about 0.01% to about 10% by weight based on the total weightof the lubricating oil composition.

The following non-limiting examples are illustrative of the presentinvention.

EXAMPLE 1

A lubricating oil composition was formed containing 3.3 wt. % of abis-succinimide (derived from a 1300 MW polyisobutenyl succinicanhydride (PIBSA)) and a mixture of heavy polyamine anddiethylenetriamine, 0.43 wt. % of a calcium sulfonate (17 BN), 3.0 wt. %of a sulfurized calcium phenate (114 BN), 0.9 wt. % of a hindered phenolantioxidant, 5 ppm of a foam inhibitor and the balance being a Group IIbase oil. The lubricating oil composition had a sulfated ash content of0.46 wt. % as determined by ASTM D 874.

COMPARATIVE EXAMPLE A

A lubricating oil composition was prepared by top-treating thelubricating oil composition of Example 1 with 0.38 wt. % of a zincdialkyldithiophosphate derived from a primary alcohol. The lubricatingoil composition had a sulfated ash content of 0.50 wt. % as determinedby ASTM D 874.

Testing

The lubricating oil compositions of Example 1 and Comparative Example Awere evaluated for exhaust valve seat recession prevention efficacy in aWaukesha F11 GSID engine. In this test, a 6-cylinder 250 BHP WaukeshaF11 GSID engine was instrumented in order to obtain dynamic voltagemeasurements (e.g., as described in U.S. Pat. No. 4,672,843) from the 12valves—6 intake and 6 exhaust valves. Each test was run for 400 hours at1800 rpm operated at 90% load. Stoichiometric conditions were maintainedduring the tests with intake air temperatures ranging between 110 to150° Fahrenheit. The average valve recession wear rates of thelubricating compositions of Example 1 and Comparative Example A werecalculated by a linear fit based on the last 300 hours of data from eachtest and reported on a wear rate per 1000 hours. The exhaust valverecession results are presented in Table 1. In this Table, the lowervalve wear recession rate represents greater exhaust valve seatrecession prevention efficacy.

TABLE 1 Waukesha F11 Exhaust Valve Recession Results Average ExhaustValve Wear Sulfated Ash Recession Rate Ex./Comp. Ex. (wt. %) (in/1000hr) 1 0.46 −0.00052 A 0.50 0.00071

As the data show, the lubricating oil composition of Example 1 exhibitedsuperior prevention of exhaust valve wear recession over the lubricatingoil composition of Comparative Example A containing a zincdialkyldithiophosphate wear inhibitor.

It will be understood that various modifications may be made to theembodiments disclosed herein. Therefore the above description should notbe construed as limiting, but merely as exemplifications of preferredembodiments. For example, the functions described above and implementedas the best mode for operating the present invention are forillustration purposes only. Other arrangements and methods may beimplemented by those skilled in the art without departing from the scopeand spirit of this invention. Moreover, those skilled in the art willenvision other modifications within the scope and spirit of the claimsappended hereto.

1. A method for preventing or inhibiting exhaust valve seat recession ina natural gas fueled engine, the method comprising lubricating theengine with a lubricating oil composition comprising (a) a major amountof an oil of lubricating viscosity; and (b) a minor amount of a naturalgas engine oil additive package, wherein the lubricating oil compositionis substantially free of each of any zinc compounds and alkaline earthmetal salts of a condensation product of an alkylene polyamine, analdehyde and a substituted phenol.
 2. The method of claim 1, wherein theoil of lubricating viscosity is a natural gas engine lubricating oil. 3.The method of claim 1, wherein the natural gas engine oil additivepackage includes (i) one or more ashless dispersants, (ii) one or moremetal-containing detergents, and (iii) one or more antioxidants.
 4. Themethod of claim 3, wherein the one or more ashless dispersants is abissuccinimide.
 5. The method of claim 4, wherein the bissuccinimideashless dispersant is derived from one or more polyalkylene succinicanhydrides.
 6. The method of claim 5, wherein the polyalkylene group isa polyisobutenyl group having an average molecular weight of from about700 to about
 2300. 7. The method of claim 3, wherein the one or moreashless dispersants is present in an amount of about 1 wt. % to about 8wt. %, based on the total weight of the lubricating oil composition. 8.The method of claim 3, wherein the one or more metal-containingdetergents is an overbased alkaline earth metal salt detergent having abase number (BN) of about 10 to about
 450. 9. The method of claim 3,wherein the one or more metal-containing detergents comprises twometal-containing detergents.
 10. The method of claim 9, wherein the twometal-containing detergents comprise a first metal-containing detergentwhich is an overbased alkaline earth metal phenate detergent having a BNof about 100 to about 450 and a second metal-containing detergent whichis an overbased alkaline earth metal sulfonate detergent having a BN ofabout 10 to about
 50. 11. The method of claim 3, wherein the one or moremetal-containing detergents is present in an amount of about 0.5 wt. %to about 8.5 wt. %, based on the total weight of the lubricating oilcomposition.
 12. The method of claim 10, wherein the firstmetal-containing detergent is present in an amount of about 0.5 wt. % toabout 5 wt. % and the second metal-containing detergent is present in anamount of about 0.1 wt. % to about 1 wt. %, based on the total weight ofthe lubricating oil composition.
 13. The method of claim 3, wherein theone or more antioxidants is a hindered phenol compound.
 14. The methodof claim 3, wherein the one or more antioxidants is present in an amountof about 0.1 wt. % to about 3 wt. %, based on the total weight of thelubricating oil composition.
 15. The method of claim 1, wherein thelubricating oil composition has a sulfated ash content of about 0.1 wt.% to about 1.5 wt. % as determined by ASTM D
 874. 16. The method ofclaim 1, wherein the lubricating oil composition has a sulfated ashcontent of about 0.15 to about 0.5 wt. % as determined by ASTM D 874.17. The method of claim 1, wherein the lubricating oil compositioncomprises: about 1 wt. % to about 8 wt. % of one or more ashlessdispersants, about 0.5 wt. % to about 8.5 wt. % of one or moremetal-containing detergents, and about 0.1 wt. % to about 3 wt. % of oneor more antioxidants, based on the total weight of the lubricating oilcomposition.
 18. A natural gas engine lubricating oil compositioncomprising (a) a major amount of an oil of lubricating viscosity, (b)one or more ashless dispersants, (c) one or more metal-containingdetergents, and (d) one or more antioxidants, wherein the lubricatingoil composition is substantially free of each of any zinc compounds andalkaline earth metal salts of a condensation product of an alkylenepolyamine, an aldehyde and a substituted phenol.
 19. The natural gasengine lubricating oil composition of claim 18, wherein the oil oflubricating viscosity is a natural gas engine lubricating oil.
 20. Thenatural gas engine lubricating oil composition of claim 18, wherein theone or more ashless dispersants is a bissuccinimide.
 21. The natural gasengine lubricating oil composition of claim 20, wherein thebissuccinimide ashless dispersant is derived from one or morepolyalkylene succinic anhydrides.
 22. The natural gas engine lubricatingoil composition of claim 21, wherein the polyalkylene group is apolyisobutenyl group having an average molecular weight of from about700 to about
 2300. 23. The natural gas engine lubricating oilcomposition of claim 18, wherein the one or more metal-containingdetergents is an overbased alkaline earth metal salt detergent having aBN of about 10 to about
 450. 24. The natural gas engine lubricating oilcomposition of claim 23, wherein the one or more metal-containingdetergents comprises two metal-containing detergents.
 25. The naturalgas engine lubricating oil composition of claim 24, wherein the twometal-containing detergents comprise a first metal-containing detergentwhich is an overbased alkaline earth metal phenate detergent having a BNof about 100 to about 450 and a second metal-containing detergent whichis an overbased alkaline earth metal sulfonate detergent having a BN ofabout 10 to about
 50. 26. The natural gas engine lubricating oilcomposition of claim 18, wherein the one or more antioxidants is ahindered phenol compound.
 27. The natural gas engine lubricating oilcomposition of claim 18, having a sulfated ash content of about 0.1 wt.% to about 1.5 wt. % as determined by ASTM D
 874. 28. The natural gasengine lubricating oil composition of claim 18, having a sulfated ashcontent of about 0.15 to about 0.5 wt. % as determined by ASTM D 874.29. The natural gas engine lubricating oil composition of claim 18,comprising: about 1 wt. % to about 8 wt. % of one or more ashlessdispersants, about 0.5 wt. % to about 8.5 wt. % of one or moremetal-containing detergents, and about 0.1 wt. % to about 3 wt. % of oneor more antioxidants, based on the total weight of the lubricating oilcomposition.
 30. The natural gas engine lubricating oil composition ofclaim 18, which has an exhaust valve seat recession reducing propertygreater than that of a corresponding natural gas engine lubricating oilcomposition in which a zinc compound is present therein.